The present invention relates generally to heating devices, and more particularly relates to control apparatus for sensing an obstruction in the combustion product exhaust portion of a fuel-fired heating appliance, such as a water heater, boiler, furnace or the like, and responsively shutting down the appliance.
Many fuel-fired heating appliances of the types mentioned above are of the forced draft variety in which a draft inducer fan is utilized to force the hot combustion gases generated by the appliance, during operation thereof, into a suitable vent pipe for discharge remote from the appliance. A common method of connecting the draft inducer fan to the appliance is to communicate the fan inlet with the outlet of a draft hood structure adapted to receive the hot combustion gases generated by the appliance and having an inlet for receiving ambient dilution air. As the draft inducer fan draws hot combustion gases through the hood it also draws ambient air into the hood. The ambient dilution air entering the hood mixes with the combustion gases in order to substantially lower their temperature before they are drawn into the draft inducer fan inlet and ultimately discharged from the fan into and through the vent pipe. This cooling of the combustion gases is particularly important in instances where a plastic material (such as, for example, PVC plastic) is used to form the vent pipe.
It is common practice to provide a heating appliance combustion gas exhaust system of this type with a safety control for detecting an obstruction in the vent pipe, which interferes with the designed-for remote discharge of the combustion gases, and responsively shutting down the appliance s that the vent pipe obstruction can be located and removed. This appliance shutdown upon a sensed vent pipe restriction serves to prevent undesirable combustion gas discharge, by reverse flow through the vent hood, immediately adjacent the appliance.
A conventional method of effecting this appliance shutdown in the event of a significant vent pipe flow restriction is to monitor the draft inducer fan scroll vacuum using a vacuum switch to prove fan operation. This is typically accomplished by connecting one end of a flexible tube or other conduit means to the outlet of the vacuum switch, and the opposite end of the tube to the fan inlet section by means of a hollow probe extending inwardly through the fan housing wall and having an open inner end positioned outwardly adjacent the fan's centrifugal impeller.
During normal operation of the combustion product exhaust system, the vacuum in the fan scroll draws a flow of ambient air into the scroll sequentially through the vacuum switch, the flexible tube and the hollow probe. This vacuum-induced inward air flow is sensed by the switch. As long as the air flow is maintained at a predetermined minimum level, the switch permits continued operation of the appliance. However, in the event that the air flow through the switch falls below such minimum level, occasioned for example by an obstruction in the vent pipe, the switch automatically shuts down the appliance.
While this vacuum switch method of sensing and responding to vent pipe obstruction has proven to be an effective and relatively inexpensive approach to monitoring vent pipe blockage, as conventionally practiced it is subject to a variety of well known limitations. For example, it is not a reliable indicator of an obstruction in the vent hood dilution air inlet. Given such obstruction, it is still possible for a negative air pressure to exist in the fan scroll of sufficient magnitude that the vacuum switch permits continued operation of the appliance. This negative pressure, though, is being achieved in this circumstance with a reduction in the intended ratio of cooling dilution air to hot combustion gases forced into the vent pipe by the draft inducer fan. The temperature of the gases discharged into vent pipe may thus be undesirably high and can damage vent pipe material.
In view of the foregoing it can be seen that it would be desirable to provide improved sensing and control apparatus that would reliably sense an obstruction in either the vent pipe or vent hood inlet opening portion of a forced draft, fuel-fired heating appliance and responsively shut down the appliance. It is accordingly an object of the present invention to provide such improved sensing and control apparatus.